The invention relates to a multi-stage side channel pump (or periphery pump) that is used to compress compressible media and whose impellers are coupled to a shared drive shaft.
In the case of multi-stage side channel pumps, the impellers of several stages, usually two stages, are normally attached next to each other on a shared drive shaft. The only constructions commonly available are those in which the impellers of two adjacent stages have the same geometry and size. The inlet of the subsequent stages is normally connected directly with the outlet of the preceding stage inside a shared housing.
German utility model no. 7,441,311 discloses a compressor configuration in which several impellers having different widths are arranged next to each other on a shared drive shaft. Each of the appertaining impellers is to be driven individually, coupled in parallel or else combined in a serial connection.
However, all of the known multi-stage side channel pumps have in common the fact that they generally do not attain optimum efficiency.
The invention provides a multi-stage side channel pump that entails optimized efficiency. According to the invention, this is achieved in that the geometry of each stage is adapted to the specific volume of the medium by determining the dimensions of the impeller and of the flow channel diameter. As a result, each subsequent stage is dimensioned altogether smaller than the preceding stage. Since the impellers of both stages rotate at the same speed, in the next stage, the pressure differential between the inlet and outlet is the same as in the stage that precedes it.
In calculating the geometry of a subsequent stage or of several subsequent stages, preferably the following approach is taken: first of all, the stage pressure p is calculated for each stage on the basis of the required final pressure and of the number of stages, according to the following relationship:
p=(pfinal)1/n
wherein
p stands for the stage pressure;
pfinal stands for the required final pressure;
n is the number of stages of a multi-stage side channel pump.
Then, the impeller geometry of the next stage is determined by means of the volume ratio of the preceding stage to the next stage, assuming geometric similarity. This volume ratio can be derived from the relationship for the adiabatic curve:
pxc2x7VK=constant.
By taking V1 and P1 for the volume and pressure of the first stage and V2 and P2 for the volume and pressure of the second stage, the following results:
P1xc2x7V1K=P2xc2x7V2K
or             V      2              V      1        =                    (                              P            1                                P            2                          )                    1        /        K              .  
If, for example, in the case of a three-stage side channel pump, the required final pressure is 2.2 bar (abs.), the result is a stage pressure p=(2.2)⅓=1.3 bar (abs).
The following applies to the volume ratio:
V2/V1=0.829, whereby the value 1.4 (diatomic gas) is taken for the adiabatic exponent K.
This ratio of 0.829 is then used to calculate the impeller geometry, assuming geometric similarity over the dimensionless characteristic line xcexa8=f("PHgr"K).
When the side channel pump according to the invention is configured with two stages, both stages are preferably integrated into one modular unit having a shared drive motor, and the impellers of both stages are attached to the shared drive shaft. If there are more than two stages, the stages are preferably lined up in a modular configuration. A particularly advantageous embodiment is one in which each pair of stages is combined in a housing to form a structural component and the impellers are attached to a shared shaft segment; the housing of one of the structural components is flanged onto the shared drive unit while another structural component, in turn, is flanged onto the first structural component, whereby the shaft segments of the structural components are each coupled to each other or to the drive shaft of the drive unit by a coupler.
Another advantageous embodiment of the invention consists in that the inlets and outlets of the stages lead to the outside separately. In this manner, the stages can be freely combined with each other or else operated separately from each other.